Overload couplings

ABSTRACT

An overload coupling has two relatively rotatable parts that are joined for corotation about a common axis by at least one frangible pin. The parts have recesses with cutting edges that sever the pins and springs that urge alternative pins on remainders of the severed pins through the recesses to automatically re-establish frangible connection. An assembly of pins can be spring loaded with the aid of a draw pin that releases the pins when in place and mounted on the coupling.

According to one aspect of the present invention there is provided anoverload coupling having a first coupling member and a second couplingmember, and a frangible pin inter-connecting the first coupling memberand the second coupling member for rotation together in normaloperation, the coupling further comprising means for moving thefrangible pin, after fracture of the frangible pin, to re-establish theinterconnection of the coupling members.

According to another aspect of the present invention there is providedan overload coupling including a spring-loaded frangible pin the springhaving a smaller diameter at the end adjacent the frangible pin than atthe end remote from the frangible pin.

The present invention also provides a drive shaft assembly including acoupling as defined above, and a holder for one or more frangible pinsfor use with a coupling as defined above.

For a better understanding of the present invention and to show how itmay be carried into effect, reference will now be made, by way ofexample only, to the accompanying drawings, in which:

FIG. 1 is an elevational view of a drive shaft assembly provided with anoverload coupling;

FIG. 2 is an enlarged elevational view of an end portion of the assemblyof FIG. 1, showing the overload coupling;

FIG. 3 is an elevational view of the coupling taken in the direction ofthe arrow III in FIG. 2;

FIG. 4 is a sectional view of the coupling taken on the lines IV--IV inFIG. 3;

FIG. 5 is a sectional view of part of the coupling taken on the linesV--V of FIG. 3;

FIG. 6 is a sectional view of part of the coupling taken on the linesVI--VI in FIG. 4;

FIG. 7 shows a holder with frangible pins suitable for use in thecoupling shown in FIGS. 1 to 6;

FIG. 8 is a sectional view of part of a second embodiment of a couplingcorresponding to the part shown in FIG. 4;

FIG. 9 is an elevational view of a third embodiment of the coupling

FIG. 10 is a sectional view of part of the coupling taken on the linesX--X in FIG. 9;

FIG. 11 is an elevational view of part of the coupling shown in FIG. 9;

FIG. 12 is a sectional view taken on the lines XII--XII in FIG. 11;

FIG. 13 is an elevational view of a fourth embodiment of the coupling;

FIG. 14 is a sectional view of part of a fifth embodiment of thecoupling; and

FIG. 15 is a sectional view taken on the lines XV--XV of FIG. 14.

The driving or intermediate shaft 1 shown in FIG. 1 comprises a two-parttelescopic main portion 2 which is pivoted at one end by means of auniversal coupling 3 to an end portion 5. The other end of the mainportion 2 is connected to an end portion 6 by a universal coupling 4.The end portion 6 comprises an overload, or shear pin, coupling 7. Theend portion 5 is hollow over part of its length and has splines whichco-operate with corresponding splines on the outer periphery of a stubshaft 8. Near the shear pin coupling 7 the end portion 6 is provided ina similar manner with axial splines 10 (FIG. 4) which co-operate withsplines 11 on the outer periphery of a power take-off shaft 9. The powertake-off shaft 9 constitutes a driving shaft and is, for example, thepower take-off shaft of an agricultural tractor, while the stub shaft 8consitutes an input shaft of, for example, an agricultural implementsuch as a soil cultivating machine.

Since the shear pin coupling 7 is a substantially symmetrical structure,only one half of the coupling will be described. The plane of symmetryis located between the two fork limbs 12 of the universal coupling 4 andcontains the rotary axis 13 of the end portion 6 and also of the shearpin coupling 7. The coupling 7 comprises a first coupling member 14having a hub 15, and a second coupling member 22. The hub 15 has thesplines 10 on its inner side. Near the universal coupling 4, the hub 15has a smaller outer diameter than it has at a distance from the coupling4. The end of the hub 15 remote from the coupling 4 is connected with aradially extending coupling plate 16; the inner diameter of the hub 15is about 20 to 30%, preferably about 24%, of the outer diameter of thecircular plate 16. The hub 15 has a radial bore 17 receiving a ball 18.The ball 18 is radially movable with respect to the rotary axis 13, butthis movement can be limited or prevented by a retaining means whichincludes a part in the form of a bolt 19. The bolt 19 has a conical endportion 20, which is adapted to co-operate with the ball 18. Thus axialmovement of the guard bolt 19 serves to retain the ball 18 in an annularrecess 21 in the end portion of the take-off shaft 9 to lock thecoupling on the shaft.

The second coupling member 22 is connected with the fork limbs 12. Thecoupling member 22 comprises a carrier 23 arranged, coaxially with therotary axis 13, between the coupling plate 16 and the fork 12. Thecarrier 23 is journalled on the hub 15 by a needle bearing 24. Thebearing 24 is provided on the part of the hub 15 having an enlargeddiameter. Near the fork 12 the carrier 23 is provided with an inner ring25 which bears, on the side facing the fork 12, on a thrust ring 26which surrounds the hub 15 and is held between the ring 25 and aretaining ring 27. The retaining ring 27 is received in a groove in thehub 15. The retaining ring 27 fixes the two coupling members 14 and 22together in the axial direction. The fork 12 is connected with thecarrier 23 by means of a retaining plate 28 and four bolts 29. The bolts29 extend parallel to the rotary axis 13 for substantially the wholelength of the carrier 23. Near the end of the carrier 23 facing thecoupling member 14 there is a centering plate 30 having a portion 31extending substantially parallel to the rotary axis 14. The bolts 29extend through the centering plate 30 and are screwed into an internalscrew-thread of a cutting ring 32. By tightening the bolts 29 the partsof the coupling member 22 can be fixed in place relatively to eachother. One side of the cutting ring 32 engages the coupling plate 16 andthe outer diameter of the cutting ring 32 is approximately equal to theouter diameter of the plate 16. The cutting ring 32 is preferably madefrom hardened material, for example, 16MnCr₅ steel. The retaining plate28 is fixed by means of the bolts 29 to a fastening part 33 of the fork12. The retaining plate 28 has, as viewed in an axial direction, asmaller radial dimension in the regions of the fork limbs 12 thanelsewhere around its circumference, and has retaining rims 34 which arebent over at right angles to the rest of the plate and are each locatedapproximately between two of the bolts 29. The rims 34 extend coaxiallywith the rotary axis 13 and are at a greater distance from the rotaryaxis 13 than is the outer surface of the carrier 23. In between the rims34 and the portion 31, which forms a second retaining rim, are slippedtwo holders 35 each containing five frangible shear pins 36 which arepreferably made from 34Cr₄ steel. Each holder 35 has a corrugated shapedouter wall, viewed along the rotary axis 13, whereas its inner wall isarcuate and coaxial with the rotary axis 13, the inner wall engaging theouter wall of the carrier 23. The inner wall of each holder 35 has aprojection or lug 37 which projects towards the fork 12 and is clampedbeneath the rim 34. At the end nearer the coupling member 14, eachholder 35 is clamped underneath the portion 31. The shear pins 36 arepushed by a spring 38 towards the first coupling member 14, the shearpins 36 being guided in a sleeve 39. The sleeve 39 has a bevelled edgeat the end nearer the spring 38 (FIG. 4) and it serves, in addition, forclosing the holder 35. The spring 38 has one portion comprising about 5to 6 coils having a diameter of about 10 to 11 mms and has anotherportion, which is in engagement with the pin 36, comprising 3 to 5 coilshaving a diameter of about 6 to 6.5 mms. On either side of the rotaryaxis 13 the centering plate 30 and the cutting ring 32 have fiveopenings having a diameter corresponding with the diameter of the shearpins 36. The springs 38 push the shear pins 36 through these openingsagainst the coupling plate 16.

The coupling plate 16 has an aperture 40 having a channel extendingradially to the outer periphery of the coupling plate. The precise widthof the channel depends on the diameter of the shear pin 36, but is about10 to 20 mms. It is highly important that the distance between the shearpin 36 in the aperture and the opposite edge of the aperture should beabout 50 to 70% of the diameter of the shear pin. In order to supportthe shear pin effectively and to break it off in the event of overload asubstantially circular cutting plate 41, preferably of hardened materialsuch as 16MnCr₅ steel, is disposed on one side of the aperture 40. Thediameter of this cutting plate is about 20 to 30 mms; in the embodimentof FIG. 6 it is about 22 mms. There are three circular cavities 42 atthe circumfernce of the cutting plate for supporting the shear pin. Thecutting plate 41 is fixed in position in the coupling plate by asecuring pin 43. Viewed in an axial direction the shear pin 36 issupported in the aperture 40 by a ledge 44 of the aperture 40, thisledge 44 supporting the shear pin over about half its head area. Theremaining area of the free end of the shear pin 36 is not supported,since radially outwardly of the edge of the ledge 44 the aperture 40 iscompletely open in an axial direction.

Before the driving shaft is put into operation, the two shear pinholders or cassettes 35 are fitted in place. The holders 35 may bedelivered to the user by the manufacturer ready for use and providedwith shear pins 36, but alternatively the user can fill the holders 35with shear pins. In order to fix the shear pins in the holder 35 aretaining member formed by a draw pin 45 is passed through the end ofthe holder remote from the spring 38 (FIG. 7), this draw pin lyingperpendicular to the longitudinal axis of the shear pins.

The holders 35 are fitted in place by first slipping the holder betweenthe retaining rim 31 and the outer wall of the carrier 23 and bysubsequently sliding the holder in the opposite sense until the lug 37snaps in between the retaining rim 34 and the fastening part 33. Thedraw pin 45 can then be withdrawn from the holder so that the action ofthe springs 38 on the shear pins and hence on the first coupling half 14automatically retains the holder between the retaining rims 31 and 34.The centering plate 30 engages the holder 35 in such a way that, whenthe draw pin 45 is withdrawn, the pins 36 slip automatically into theopenings of the cutting ring 32. In this manner the retaining rims 31and 34 together with the lug 37 co-operate to provide a quick-actionconnection for the holder 35 under the action of the spring 38. By aturning the two coupling members 14 and 22 relatively to each other, oneof the shear pins will click into the opening 40 to interconnect the twocoupling members. After the power take-off shaft 9 has been secured withthe shear pin coupling by the retaining bolt 19 and the ball 18 andafter the end portion 5 has been connected with the stub shaft 8, thedriving shaft 1 is ready for use.

If the operation of the implement connected with the stub shaft 8 isobstructed in some way and there is consequently a danger that theimplement and the driving shaft 1 will be overloaded, the portion of theshear pin 36 located in the opening 40 will break off. The supportingring 32 and the cutting plate 41 ensure a clean cut of the pin 36 sothat the new free end of the shear pin 36 has a smooth surface. Becausehardened material is used for the cutting plate 41 and the supportingring 32, damage of the further parts of the coupling member 14 and 22 isavoided. The broken-off fragment of the shear pin 36 is ejected in aradial and, as in some cases, axial, direction owing to the shape of theopening 44, so that the opening 40 is cleared very quickly. Since thetwo coupling members are rotatable relatively to one another due to thebearing 24, and since the two coupling members are held together axiallyby the retaining ring 27, the two coupling members can rotate freelywith respect to one another after fracture of the pin. Owing to thecritical tangential magnitude of the opening 40, re-interconnection ofthe two coupling members, that is to say, resetting of the shear pin 36in a connecting position, can only be performed at a lower speed thanstalling speed of the power take-off shaft. This means that the powertake-off shaft has to be intentionally disconnected before the shear pincoupling can be re-connected. The tangential magnitude of the opening 40is determined by the factor T in the formula S=1/2AT², in which Srepresents the distance by which the pin must advance towards theopening 40 to establish the connection between the two coupling members.During use the terms A and T are approximately constant for each pindiameter. In order to maintain the term A constant, the pressure appliedby the spring 38 has to be proportional to the length of the pin 36. Inthis way the shear pin coupling can be re-connected automatically. Sincethe pins 36, when new, have a length of, preferably, about 30 to 60 cms,they can break off many times, and so they can be used for a long periodfor repeatedly re-connecting the two coupling members. Because there isa plurality of cavities 42 in the cutting plate 41, this plate can beset in a new position when one cavity 42 become worn or otherwisedefective, so that a new cavity 42 can be used for engaging the shearpin in the connecting position. The coupling can be adjusted for thetransmission of higher or lower power by using holders 35 withcorrespondingly stronger or weaker shear pins and by replacing thecutting ring 32 and the centering plate 30 by others having apertures ofthe right size for the shear pins. The design of the spring 38 is highlyimportant for ensuring the correct re-connecting effect of the shear pin36 as described above. It is also very advantageous that the diameter ofthe spring at the end which engages the shear pin is smaller than at theother end. Because of this feature, the spring 38 can push the shear pin36 through the guide sleeve 39 to increase the number of times which thepins 36 can break off.

FIG. 8 shows an embodiment of a holder or cassette 46 which can be usedin the coupling 7 of the preceding Figures. The holder 46 has anexternal sheath substantially similar to that of the holder 35. At thetop the holder is closed by means of a closing cap 47. There may be aseparate closing cap 47 for each pin, but alternatively the closing capsmay be united to form an uninterrupted closing cover for all of theshear pins in the holder. The closing cap is preferably made from asynthetic material and has a partly annular rib 48 for fixing theclosing cap 47 in the inner wall of the holder 46. The closing cap 37has a cavity which reveives one end of a compression spring 49 providinga force approximately the same as that of the spring 38 of the firstembodiment. The end of the spring 49 remote from the cap 47 is receivedin a cavity provided in a plunger 50. The plunger 50 pushes home theshear pin 51. In order to guide the plunger 50 the holder 46 has rigidlysecured to it a guide sleeve 52. In order to allow a displacement of theplunger 50 over a sufficient distance through the sleeve 52, itsdiameter over about 60% of its length is smaller than that over the restof its length. The plunger 50 may be made of a synthetic substance, sothat a smooth displacement along the wall of the holder 46 is ensured,but it has been found to be advantageous it make the pressure pin 50from fairly heavy material in order to resist to some extent undesirabletilting or jerking movements of the shear pin 51. The holder 46 can befilled in a very simple manner with shear pins 52, while with thisconstruction each shear pin 52 is satisfactorily and uniformly actedupon over the whole of the surface of its free end. The smaller diamterportion of the plunger 50 means that the shear pin can be pressed hometo near the aperture in the cutting ring 32. The residual portion of thebreaking pin, which can finally no longer be utilized, is thusminimized.

FIGS. 9 to 12 show a third embodiment of a shear pin coupling 77. Partscorresponding with those of the preceding Figures are designated by thesame reference numerals. A first coupling member 53 comprises, asbefore, the coupling plate 16 and the hub 15. By means of the retainingring 27 shown in FIG. 4, the frist coupling member 53 is secured in theaxial direction to a second coupling member 54. The second couplingmember 54 is connected, preferably by six bolts 56, with a fasteningpart 55, which comprises a substantially circular plate connected withfork limbs 57 of the universal joint 4. To the fastening part 55 issecured a carrier 58, the function of which is comparable to that of thecarrier 23 of FIG. 4. The carrier 58 is disposed between the fasteningpart 55 and the cutting ring 32. As shown in the sectional view of FIG.10 the carrier 58 comprises an inner ring 59 similar to the inner ring25 in FIG. 4, for fixing the carrier in place with respect to the firstcoupling member 53. The carrier 58 has two cavities or recesses 60,positioned diametrically opposite each other with respect to the rotaryaxis 13, for holding a plurality of shear pin holders. Each cavitypreferably comprises 5 cylindrical holders 61 each containing a shearpin 62. Each holder 61 comprises a tube 63 having a diameter of about 16mms. The tubes 63 may be made of a synthetic material such as plastics.Alternatively, the tubes 63 may be made from metal. Each tube 63 coversthe whole axial distance between the fastening plate 55 and the cuttingring 32. Near the cutting ring 32 the tube is provided with a taperinginner ring 64. The smaller diameter part of this ring 64 consitutes aguide for the shear pin 62. The tube 63 is closed at its end nearer thefastening plate 55 by a plug or cap 65, which is rigidly secured to thetube 63 by means of a tensile pin 66. The plug 65, which is preferablymade from a synthetic substance, has a cavity, on the side facing theinterior of the tube 63, receiving one end of a compression spring 67.The compression spring 67 approximately corresponds with the spring 49of FIG. 8. At its end remote from the plug 65, the spring 67 is receivedin a plunger 68, one end of which is substantially in full engagementwith the free end of the shear pin 62. The plunger has a portion ofenlarged diameter which engages the inner wall of the tube 63. Theplunger 68 may have a fairly considerable mass to avoid tilting orjerking of the shear pin 62. The shear pins and their holders 61 areretained in the cavity 60 by a sliding cover 69. The sliding cover 69has an arcuate shape centered on the rotary axis 13 and has on two sidesaxially bent-over rims 70. The rims 70 co-operate with clamping springs71. Each clamping spring 71 has several turns 72 surrounding one of thebolts 57 and is arranged in a recess 73 in the carrier 58. The recess 73is completely open in a radially outward direction and is bounded oneach end, as viewed axially, by walls 74 of the carrier 58. The clampingspring 71 has an end portion 75, extending away from the cavity 60,which is in engagement with a radially extending bulging part 76 of thecarrier 58 surrounding a bolt 56; this bulging part 76 separates twoadjacent recesses 73 from one another. The spring 71 also has a portion77 bent over through about 180°. The free end of this portion 77 pressesthe adjacent end portion of the bent-over rim 70 towards the rotary axis13. As shown in the elevational view of FIG. 9, the bent-over portion77, comprising two limbs interconnected by a web, bears by one of thelimbs on a locally pressed out part of the rim 70. The sliding cover 69has a second pressed out part 78 on the other side of the clampingspring 71 so that the cover 69 can be slid, by means of bent-over sides79 or 79A, in the direction of the arrow A across the cavity 60.

In this embodiment the coupling plate 16 also has an opening 40 forreceiving a shear pin 62 in a connecting position. In order to supportthe shear pin 62, a cutting plate 80 is recessed into the plate 16, thiscutting plate 80, like the plate 41, being made from hardened material.The cutting plate 80 has at its circumference four cavities 81 forreceiving the portion of the shear pin 62 projecting into the opening40. The cavities 81 (see FIG. 11) are arcuate with a radius of about 6mms. The cutting plate 80 is rigidly secured to the plate 16 by means ofa bolt 82, which is fixed in place on the side of the plate 16 remotefrom the second coupling member 54 by a self-locking nut acting on oneor more cup springs 83. On the one side of the opening 40 remote fromthe cutting plate 80 the plate 16 has a recess for receiving a fillingplate 85. The filling plate 85 is fixed to the plate 16 by a bolt and anut 86. On the basis of the dimension indicated by 87 in FIG. 11 being16 mms, the filing plate 85 has the sizes 88 and 89 as follows:

    ______________________________________                                        Diameter                                                                      of shear pin (mm)                                                                         Dimension 88 (mm)                                                                            Dimension 89 (mm)                                  ______________________________________                                        7           20.0           28.0                                               8           18.5           26.5                                               10          15.5           23.5                                               ______________________________________                                    

The construction shown in FIGS. 9 to 12 operates as follows.

With the cover 69 removed, the cavity 60 of the shear pin coupling 7 canbe filed with shear pin holders 61 having each a shear pin 62. In orderto facilitate this operation a draw pin as shown in FIG. 7 may be used.The holders 61 are simple of construction and hence quite cheap so thatthe user can keep a supply of spare holders in stock. When the cover 69is slid across the cavity 60 and thus the rim 70 is clamped tightlybeneath the bent-over part 70, the cover 69 is retained in place by theengagement of the pressed out part 78 with the portion 77. The bent-oversides 79 and 79A facilitate the positioning of the cover 69. The furtheroperation of the coupling corresponds with the operation described forthe preceding embodiments. After rupture of the shear pin located in theopening 40 the speed of the power takeoff shaft has first to be reducedas in the preceding embodiments. In the present embodiment the factor Tof the formula S=1/2AT², which factor is, in fact, determined by thediameter of the pin and the associated tangential magnitude 87 for theopening 40, can be kept constant with various diameters of the pin,since filling plates 85 of different dimensions 88 and 89 can be used independence upon the diameter of the pin. As shown in FIG. 11, thecutting plate 80 is adjustable by simply turning it about the bolt 82 topresent a different cavity 81 in the opening 40 so that after any damageof one cavity 81 the cutting plate 80 can be readily moved to present anew cavity 81. The cup springs 83 absorb shocks on the plate 80.

FIG. 13 shows a further embodiment of shear pin coupling 7, in whichthree holders 35 are disposed between the retaining rim 31 and aslightly matching retaining rim 87A. The retaining rim 87A clamps thefastening part 33 and the fork limbs 12 with the carrier 23 tightlyagainst the cutting ring 32 by means of bolts 88A. The three holders 35further raise the capacity of the coupling 7, which can thus be used foreven a longer time without needing to replace shear pins than in thepreceding embodiments.

FIGS. 14 and 15 show an alternative form of the embodiment shown in FIG.8, in which a shear pin 89A has a groove 90 extending over its wholelength and parallel to its longitudinal center line. This groove 90co-operates with corresponding lugs on the guide sleeve 52, thecentering plate 30 and the cutting rings 32. It has been found to beadvantageous to located the breaking pin so as to be non-rotatable inthe opening 40. This is ensured by the groove 90 and the correspondinglugs on the further parts of the coupling. It is thus ensured that, whenthe shear pin portion located in the opening 40 braks off, the cuttingeffect on the breaking pin is minimized, so that a sharply definedbreaking surface is formed each time, which enhances thedisturbance-free use of the pin 89A for a long time. This shape for thebreaking pin 89A is highly advantageous for the multi-rupture shear pinused in the holders described above.

While various features of the coupling that have been described, andthat are illustrated in the drawings, will be set forth in the followingclaims as inventive features, it is to be noted that the invention isnot necessarily limited to these features and that it encompasses all ofthe features that have been described both individually and in variouscombinations.

What we claim is:
 1. An overload coupling comprising a first member anda second member that are drivenly interconnected by fragible means forrotation together about a common axis during normal operation, saidfragible means comprising at least one fragible pin that interconnectsthe coupling members and said pin being fitted in cutting means thatsevere the pin upon overload to permit relative rotation between saidmembers, means urging said fragible means into said cutting means tore-establish an interconnection between said coupling members, said pinbeing supported in a holder connected to the coupling by a quick-actionconnection.
 2. An overload coupling as claimed in claim 1, in which thequick-action connection is urged by a spring into operative position. 3.An overload coupling as claimed in claim 2, in which said spring ishoused in the holder and bears on one end of the pin.
 4. An overloadcoupling as claimed in claim 3, in which the spring is a compressionmember and urges the holder away from one of said coupling members. 5.An overload coupling as claimed in claim 4, in which the holder isclamped by the spring and the latter is positioned between two retainingplates of said coupling.
 6. An overload coupling as claimed in claim 5,in which a projection of the holder cooperates with one of said plates.7. An overload coupling as claimed in claim 6, in which one end of thespring faces the pin, and that end has a smaller diameter than theopposite end of the spring.
 8. An overload coupling comprising a firstmember and a second member that are drivenly interconnected by fragiblemeans for rotation together about a common axis during normal operation,said fragible means comprising a plurality of frangible pins thatslideably house within at least two holders and each holder isreleaseably connected to one of the coupling member, at least one of thepins being urged into cutting means located adjacent an interfacebetween said coupling members, an operative position of said pinengaging the other coupling member and that portion being severed uponoverload to permit relative rotation between the two members, springmeans selectively urging any one of the pins into the cutting means tore-establish an interconnection between the coupling members.
 9. Anoverload coupling as claimed in claim 8, in which the holders areconnected to said one coupling member by centering means.
 10. Anoverload coupling as claimed in claim 9, in which the holders arepositioned coaxially with the center line of the coupling.
 11. Anoverload coupling as claimed in claim 9, in which the holders aresymmetrically located with respect to the axis of rotation of thecoupling.
 12. An overload coupling as claimed in claim 8, in which eachholder has a cylindrical outer circumference.
 13. An overload couplingas claimed in claim 8, in which at least part of the surface of eachfrangible pin is shaped to cooperate with a correspondingly shaped partof the cutting means.
 14. An overload coupling as claimed in claim 13,in which said pin is uniformly shaped.
 15. An overload coupling asclaimed in claim 8, in which each holder houses five frangible pins. 16.An overload coupling as claimed in claim 8, in which the frangible pinsare retained in place in their respective holders by a removable drawpin.
 17. An overload coupling as claimed in claim 16, in which the drawpin extends perpendicular to the longitudinal axis of the pins.
 18. Anoverload coupling as claimed in claim 8, in which each holder is mountedon an axial extending carrier of one coupling member.
 19. An overloadcoupling as claimed in claim 8, in which a cutting ring is positioned atsaid interface between the two coupling members and said ring isreleasably fixed to said one coupling member.
 20. An overload couplingas claimed in claim 19, in which the cutting ring has at least oneaxially extending aperture.
 21. An overload coupling as claimed in claim20, in which each frangible pin projects into a respective aperture. 22.An overload coupling as claimed in claim 8, in which the first has anopening that receives the pin to establish interconnection between themembers and said opening having a limit end, said pin extendingsubstantially parallel to the axis of rotation.
 23. An overload couplingas claimed in claim 22, in which the opening opens radially outwardly.24. An overload coupling as claimed in claim 23, in which a tangentialside of the opening is bounded on one side by a displaceable cuttingplate in the first member.
 25. An overload coupling as claimed in claim24, in which the periphery of the cutting plate is at least partlydefined by arcs of a common circumscribing circle.
 26. An overloadcoupling as claimed in claim 25, in which the cutting plate is connectedto the first coupling member near the center of the circumscribingcircle.
 27. An overload coupling as claimed in claim 25, in which thecutting plate is connected near its outer periphery to the first member.28. An overload coupling as claimed in claim 27, in which a plurality ofcavities for receiving a frangible pin are located adjacent theperiphery.
 29. An overload coupling as claimed in claim 28, in which thecutting plate is fixed in position with a retaining pin that engages oneof the cavities.
 30. An overload coupling as claimed in claim 24, inwhich the tangential magnitude of said opening is adjustable.
 31. Anoverload coupling as claimed in claim 30, in which said tangentialmagnitude is about 20% to 60% of the diameter of the frangible pin. 32.An overload coupling as claimed in claim 24, in which the axialdimension of the opening increases in a radially outward direction. 33.An overload coupling comprising a first member and a second member thatare interconnected to one another by a frangible pin for rotation abouta common axis, said first member including a circular hub that isjournalled within an encircling carrier of the second member and acutting ring encircling the carrier, said pin extending through anopening in said ring and into engagement with an aperture of plate meansof said first member, said plate means overlying the hub and including acutting member that together with the ring, severs the pin upon overloadand permits the relative rotation of one member relative to the other.